\documentclass[red]{mytest}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage{blindtext}
\title{Example to show how classes work}
\author{Team Learn ShareLaTeX}
\date{2019/4/20}

\begin{document}
\maketitle
\noindent
Let's begin with a simple working example here.
\blindtext
\section{Introduction}
The Monty Hall problem...
\blindtext
\section{The same thing}
The Monty...
\blindtext
\section{The same thing}
The Monty...
\blindtext

\section{The same thing}
The Monty...
\blindtext

\section{The same thing}
The Monty...
\blindtext

\end{document}